1. Field of the Invention
The present invention relates to a magneto-optic recording medium for recording information by applying a light beam and a magnetic field thereto, a method of manufacturing the magneto-optic recording medium, and a magneto-optic recording system for recording information on the magneto-optic recording medium. More particularly, the present invention concerns a magneto-optic recording medium wherein a first magnetic layer whose direction of magnetization is reversible and a second magnetic layer whose direction of magnetization is irreversible are laminated on a substrate, a method of manufacturing the magneto-optic recording medium, and a magneto-optic recording system for recording information on the magneto-optic recording medium.
2. Description of the Related Art
FIG. 12 is an enlarged cross-sectional view of a conventional magneto-optic recording medium in which a single magnetic layer is laminated on a substrate. A magneto-optic recording medium D comprises a substrate 2, formed of glass or a plastic, and a single magnetic layer 1 formed on one surface of the substrate 2. This magnetic layer 1 exhibits vertical magnetic anisotropy, and if a light beam is applied to the magnetic layer 1 so as to increase its temperature up to its Curie temperature, the direction of magnetization of the magnetic layer 1 can be set in correspondence with the direction of a magnetic field applied from the outside to the magneto-optic recording medium D. That is, by reversing the direction of the magnetic field applied to the magneto-optic recording medium D in connection with the recording or erasing of information, it is possible to record information on the magneto-optic recording medium D.
FIG. 13 is a schematic enlarged cross-sectional view of another conventional magneto-optic recording medium D. The magneto-optic recording medium D comprises two unit recording media A, B. One (the other) unit recording medium A (B) is constituted by a substrate 2A (2B) formed of glass or a plastic and a magnetic layer 3A (3B) formed on one surface of the substrate 2A (2B). The one unit recording medium A and the other unit recording medium B are formed integrally by being laminated on each other with their respective magnetic layers 3A, 3B abutting each other via an adhesive layer AD. Both of these magnetic layers 3A, 3B exhibit vertical magnetic anisotropy and are arranged such that if a light beam is applied to the magnetic layers 3A, 3B so as to increase their temperatures up to their Curie temperatures, the direction of magnetization of the magnetic layers 3A, 3B is reversed in correspondence with the direction of the magnetic field applied from the outside to the magneto-optic recording medium D. That is, by reversing the direction of the magnetic field applied to the magneto-optic recording medium in connection with the recording or erasing of information, it is possible to record or erase information with respect to the magneto-optic recording medium D.
FIGS. 14 and 15 are schematic perspective views of magneto-optic recording systems for recording information on the above-described magneto-optic recording medium D. In FIG. 14, a light-emitting element LD for emitting a light beam is disposed on one surface of the magneto-optic recording medium D in the vicinity of an edge thereof, while a permanent magnet MG capable of reversing a magnetic pole surface by means of a motor M is disposed on the other surface thereof in the vicinity of an edge thereof so as to apply its magnetic field to the magneto-optic recording medium D. The motor M is arranged to be driven in response to an output of a driving circuit DR to which a signal concerning the recording or erasing of information is inputted.
When effecting the recording or erasing of information by means of such a magneto-optic recording system, the magneto-optic recording medium D is first driven. Then, a light beam LB is projected onto the magneto-optic recording medium D, and a signal concerning the recording or erasing of information is inputted to the driving circuit DR. The temperature of the portion of the magneto-optic recording medium D to which position the light beam LB has been projected becomes high owing to the projection of the light beam LB thereto, whereas there occurs a drop in the temperature of the portion of the magneto-optic recording medium D to which the light beam LB is no longer projected owing to the rotation of the magneto-optic recording medium D. Furthermore, the rotating direction of the motor M changes in relation to the signal concerning the recording or erasing of information as indicated by the arrows. In other words, the direction of a magnetic field which the permanent magnet MG applies to the magneto-optic recording medium D reverses. The magnetic layer 1 whose temperature has reached the Curie temperature is magnetized in the direction of the magnetic field being applied at that time. For instance, when erasing information, the magnetic layer 1 is magnetized in an opposite direction to the direction of magnetization of the substrate 2. Meanwhile, when recording information, the magnetic layer 1 is magnetized in the same direction as that of the substrate 2.
In FIG. 15, the light-emitting element LD is disposed on one surface side of the magneto-optic recording medium D in the vicinity of an edge thereof in the same way as shown in FIG. 14. Meanwhile, an electromagnet in which an exciting coil WD is wound around an elongated rectangular magnet yoke MY is disposed on the other surface side thereof in the vicinity of an edge thereof and is arranged to apply a magnetic field to the magneto-optic recording medium D. An output of the driving circuit DR to which the signal concerning the recording or erasing of information is inputted is applied to the exciting coil WD. The magneto-optic recording system thus arranged excites the exciting coil WD in connection with the recording or erasing of information by means of the driving circuit DR, thereby reversing the direction of the magnetic field applied to the magneto-optic recording medium D in correction with the recording or erasing of information. Then, information is recorded in the magnetic layer 1 through the same operation as that of the magneto-optic recording system shown in FIG. 14.
Meanwhile, a magneto-optic recording medium D shown in FIG. 16 has been developed to overcome the drawback encountered in reversing the direction of a magnetic field applied to the magneto-optic recording medium described above. This magneto-optic recording medium D comprises the substrate 2 formed of glass or a plastic, a first magnetic layer 3 formed on one surface of the substrate 2 and a second magnetic layer 4 laminated on the first magnetic layer 3. Both the first and second magnetic layers 3, 4 exhibit vertical magnetic anisotropy, and the first magnetic layer 3 is coupled with the second magnetic layer 4 with an exchange coupling force acting therebetween. This second magnetic layer 4 is magnetized in one direction and its direction of magnetization is irreversible, while the direction of magnetization of the first magnetic layer 3 is reversible. In addition, the Curie temperatures T.sub.C1, T.sub.C2 of the first and second magnetic layers 3, 4 are set in such a manner that T.sub.C1 &lt;T.sub.C2. If it is assumed that amounts of shift of a reversed magnetic field obtained from the exchange coupling force acting between the first and second magnetic layers 3, 4 are H.sub.W1, H.sub.W2, and the bias magnetic field (Hb&gt;0) at the time of recording information is H.sub. b, then coersive forces H.sub.C1, H.sub.C2 of the first and second magnetic layers 3, 4 at ordinary temperature are provided as H.sub.C1 &gt;H.sub.W1 +H.sub.b, H.sub.C2 &gt;H.sub.W2 +H.sub.b, respectively.
When recording information on the magneto-optic recording as described above, it is necessary to reverse the direction of magnetization of the magnetic layer 1 by reversing the direction of the magnetic field being applied to the magneto-optic recording D. For that reason, when reversing the magnetic polarity of the permanent magnet MG by using the motor M, a space for providing the motor M and a space for rotating the permanent magnet MG are required, and driving electric power for the motor M is required. Furthermore, the direction of the magnetic field reverses at low speed.
Meanwhile, when reversing the direction of the magnetic field by reversing the direction of flow of electric current for the exciting coil WD, it is possible to reverse the direction of the magnetic field at high speed. However, since the exciting coil WD is large in size with an elongated rectangular configuration, large exciting electric power is required, and a space for preventing the generation of heat is also required. Accordingly, with either of the structures described above, there has been the problem that it is impossible to reduce power consumption of the magneto-optic recording system and make the system compact in size.